1. Field of the Invention
The present invention relates to an oscillation circuit, and more particularly to an oscillation circuit of phase-shift type employing a surface acoustic wave device.
2. Description of the Related Arts
Oscillation circuits with less noise which employ a resonator having a large Q value are used in various fields of electronic industries. There are two types of oscillation circuits: one is an oscillation circuit with a fixed frequency, and the other is an oscillation circuit of voltage controlled type whose oscillation frequency can be varied with an external voltage (hereafter referred to as VCO: Voltage Controlled Oscillator). Oscillation circuits of voltage controlled type are used in various electronic apparatus and, recently, there has been a need for oscillation circuits of voltage controlled type having a high C/N ratio with a large bandwidth of variable frequency especially in the fields of mobile communication and optical communication. Particularly, it is required that oscillation circuits of voltage controlled type demanded in the mobile communication terminal such as a car phone or a portable phone are provided in a smaller size and with lower costs. In view of this, oscillation circuits of phase-shift type are a prospect among the oscillation circuits of voltage controlled type.
Conventionally, oscillation circuits employing a dielectric resonator or VCOs employing a micro strip line as a resonator are used in the field of mobile communication. However, the former is provided in a large size with high costs, and therefore is not suitable as a VCO for mobile communication. The latter can be provided in a small size with low costs and can be furnished with a large bandwidth of variable frequency, but it cannot have a large C/N ratio because the Q value is low.
Also, there were reports on VCOs employing a surface acoustic wave resonator (SAW) or a surface acoustic wave filter (hereafter referred to as a SAW filter). Oscillation circuits employing a quartz as a material for the substrate of a SAW filter have a high Q value and excellent C/N ratio but the frequency range that can be controlled with voltage is narrow.
In order to overcome the aforementioned problems, oscillation circuits are proposed in which a piezoelectric single crystal having a large electromechanical coupling factor is used as a substrate material. For example, there have been reports on an oscillation circuit employing LiTaO.sub.3 ("Surface acoustic wave oscillation circuits with a large bandwidth of variation" (Kouichi Sugawara and Bunji Yamamoto, Technical reports by Electronic Data Communication Society, US78-12, pp. 21-28) and on an oscillation circuit employing LiNbO.sub.3 ("Review on voltage controlled oscillation circuits for mobile wireless transceivers employing a SAW resonator" (Mitsutaka Hikida, Junji Sumioka, and Yukinari Fujiwara, Technical reports by Electronic Data Communication Society, MW90-102, pp. 15-20).
However, conventional VCOs employing a SAW device (a SAW filter or a SAW resonator) having a substrate with a high electromechanical coupling factor are accompanied by the following problems.
(1) A VCO having a one-port SAW resonator formed on a LiNbO.sub.3 substrate
FIG. 21 shows a structural view of a conventional one-port SAW resonator. The one-port SAW resonator includes an IDT (interdigital transducer) and reflectors. The IDT is disposed at the center of the resonator and composed of finger-like electrodes. The reflectors are disposed on both sides of the IDT. FIG. 22 shows impedance-frequency characteristics of this one-port SAW resonator, in which the vertical axis represents an imaginary part of the impedance, fr being a resonance frequency and fa being an anti-resonance frequency.
FIG. 23 is a circuit diagram of a VCO employing a one-port SAW resonator. In principle, this VCO has a variable frequency within the range from the resonance frequency fr to the anti-resonance frequency fa of the resonator, as shown in FIG. 22. The frequency difference fa-fr is determined by the capacitance ratio .gamma. of the resonator and is a value substantially determined by the electromechanical coupling factor of the substrate material (piezoelectric single crystal) for the SAW resonator. In other words, the difference fa-fr increases in accordance with the increase in the electromechanical coupling factor, whereby the bandwidth of variable frequency as a VCO will be larger.
Therefore, a variable frequency bandwidth of about 2.5% in terms of fractional bandwidth is conventionally realized by employing a LiNbO.sub.3 substrate having a large electromechanical coupling factor. However, a variable frequency bandwidth of about 3 to 3.5% in terms of fractional bandwidth is required in the field of mobile communication, especially of portable phones and wireless LAN, so that the conventional VCO could not be used in this field.
(2) A VCO employing a transversal SAW filter formed on a LiTaO.sub.3 substrate.
FIG. 24 shows a circuit diagram of a VCO employing a conventional transversal SAW filter. This VCO has a transversal SAW filter formed on a LiTaO.sub.3 substrate as shown in FIG. 24. The SAW filter is disposed at the feedback circuit portion of a phase-shift oscillation circuit for obtaining an oscillation. In this case, it is possible to enlarge the variable frequency bandwidth in principle by reducing the number of electrode pairs of the transversal filter and decreasing the delay distance between the input and output electrodes.
However, there is a problem that ripple in band inevitably occurs due to the TTE wave (Triple Transit Echo: the delayed wave generated by triple reflection in the SAW filter between the input and output electrodes), thus failing to satisfy the condition for stable oscillation. Also, in this example, the reduction of the number of electrode pairs is limited in view of preventing the side lobe, so that the obtained bandwidth of variable frequency was a fractional bandwidth of only about 2.5%.